1. Field of the Invention
The present invention relates generally to a shim attaching/detaching tool for a tappet of an internal combustion engine. More particularly, the present invention relates to a shim attaching/detaching tool for properly adjusting a valve clearance for a tappet arranged in a direct-acting type valve driving mechanism for an internal combustion engine.
2. Description of the Related Art
To facilitate understanding of the present invention, a direct-acting type valve driving mechanism hitherto employed mainly for a DOHC type engine will be described below.
FIG. 7 is a fragmentary sectional view which schematically shows the structure of a direct-acting type valve driving mechanism. The valve driving mechanism includes an engine valve 101 of which valve section is not shown in the drawing and which is fitted into a cylinder head 105. In addition, a pair of cotters 102, a spring retainer 103 and a valve spring 104 are arranged for the engine valve 101.
As shown in the drawing, a cylindrical tappet 106 of which upper surface is closed with an upper wall 107 is inserted through the cylinder head 105 from above while the bottom surface of the upper wall 107 comes in contact with the uppermost end of the engine valve 101.
The upper wall 107 of the cylindrical tappet 106 includes an annular side wall 108 which serves to define a circular cavity 109 so as to allow a shim 110 for adjusting a valve clearance to be fitted thereinto.
The engine valve 101 is adapted to open and close as the tappet 106 is vertically displaced by a rotary cam 111 which is normally brought in contact with the upper surface of the shim 110.
With the direct-acting type valve driving mechanism constructed in the above-described manner, adjustment of the valve clearance is achieved by exchanging the shim 110 with another one having a different thickness while the tappet 106 is depressed by actuating a certain jig (not shown). However, since a quantity of projection of the shim 110 above the upper end of the tappet 106 is small, a gap around the fitting portion of the shim 110 is minimized as far as possible, and moreover, the shim 110 comes in close contact with the upper surface of the upper wall 107 while a film of lubricant is interposed therebetween, it is very difficult to detach the shim 110 from the tappet 106.
To cope with the foregoing difficulty, proposals have been made with respect to tappets as shown in FIG. 8 and FIG. 9.
In the case of a tappet 106 shown in FIG. 8, a pair of slit-shaped cutouts 112 are diametrically formed on an annular side wall 108 which projects above an upper wall 107 of the tappet 106. When a shim 110 is exchanged with another one having a different thickness, a tool such as a driver or the like is inserted into each cutout 112 so as to detach the shim 110 from the circular cavity on the upper wall 107 of the tappet 106.
In case of the tappet 106 shown in FIG. 9, an aperture 113 is axially formed through a shim 113 so that a rod-shaped detaching jig (not shown) is inserted into the aperture 113 so as to allow the shim 113 to be raised up away from an upper wall 107 of the tappet 106 to exchange the shim 110 with another one having a different thickness or a compressed air is blown through the aperture 113 so as to allow the shim 110 to be floated up away from an upper wall 107 to exchange the shim 110 with another one.
In recent years, there is recognized a general tendency that tappets each made of a light metallic material such as aluminum alloy or the like are substituted for those made of a ferrous metallic material such as cast iron or the like for the purpose of designing a valve driving system with smaller weight.
Since a tappet made of a light metallic material is inferior to that made of a ferrous metallic material in respect of strength and rigidity, there arises a malfunction that the valve driving system incorrectly operates due to the diametrical formation of the cutouts 112 on the side wall 108 as shown in FIG. 8 during rotation of the engine at a high speed. Thus, when the side wall 108 is loaded with a large magnitude of repeated load effective in the outward direction, there is a possibility that the side wall 108 is damaged or broken due to the crack caused by the stress concentrated on each cutout 112. It should be added that the number of machining steps increases corresponding to the formation of the cutouts 112, resulting in the tappet 106 being produced at an increased cost.
On the other hand, in case of the tappet shown in FIG. 9, the number of machining steps likewise increases corresponding to the formation of the aperture 113 through the shim 110, resulting in the tappet 106 being disadvantageously produced at a high cost. The shim 110 is caused to rotate in a certain direction by the action of a force effective at a right angle relative to the axis line during rotation of the rotary cam 111. Thus, the face pressure appearing on a slidable contact face between the shim 110 and the rotary cam 111 intermittently varies every time the nose face of the rotary cam 111 is brought in contact with the aperture 113, moreover, the face pressure is raised up to be higher than that in the surrounding region.
Consequently, local wear occurs not only in the surrounding region of the aperture 113 on the upper surface of the shim 110 but also on the nose face of the rotary cam 111, resulting in durability of the tappet 106 being degraded.